This invention relates to a novel and simple method of producing a stamped plastic sheet of high utility in which a thermoplastic plastic film or sheet (hereinafter referred to merely as a "sheet") is permanently stamped, and to a novel and useful stamped plastic sheet provided by the method.
In general, in stamping a plastic sheet, a sheet-shaped or cylinder-shaped molten thermoplastic material extruded from the nozzle of a T-die, screw die or circular die is allowed to pass through the clearance or gap between stamping rolls such as embossing rolls, so as to be pressed while being heated, kept warm or cooled. Especially in manufacturing an uneven, wavy sheet, the sheet should be stamped while the core of the sheet is soft. Accordingly, in the case of the thin sheet, the temperature must be strictly controlled before it is stamped. On the other hand, in the case of the thick sheet, two contradictory conditions must be satisfied; that is, while it requires substantial time to soften the sheet to the core, the stamped sheet must be quickly cooled down to its heat distortion temperature. Thus, stamping a thick sheet is more difficult than stamping a thin sheet. In stamping a thick sheet, it is necessary to reduce the stamping speed to an uneconomical speed, or to provide an additional means such as special cooling rolls. In the latter case, the cooling rolls must be precise, and an additional mechanical problem, namely the synchronization in pitch of the unevenness of the cooling rolls is involved.